Project Summary Adult neurogenesis not only has critical roles in memory formation and mood regulation, but it also offers the potential for regeneration of neurons and function lost to brain injury. Accordingly, neurogenesis increases after many types of injury, including cerebral ischemia/reperfusion injury during cardiac arrest (CA) and resuscitation. The cellular mechanisms behind the induction of post-ischemic neurogenesis remain unclear, creating a critical barrier to new interventions that could support and sustain neurogenesis to improve recovery. Brain ischemia causes profound and sustained activation of microglia, the brain resident immune cells, which exacerbates neuronal death and has been implicated as a negative regulator of neurogenesis. Using a mouse model that allows us to selectively remove brain microglia, we obtained strong preliminary data supporting an unexpected, critically supportive role of microglia for neurogenesis after CA. In the proposed study, we will expand on these findings and define whether microglia affect proliferation, maturation, and long-term survival of newborn neurons after CA. Aim 1 will test the hypothesis that microglia support proliferation and survival of newborn neurons in the dentate gyrus after global ischemia, and further define the supportive microglial phenotype. Aim 2 will test the hypothesis that microglia support functional integration of newborn neurons after global ischemia. We will ablate microglia in the early peri-ischemic phase as well as during late recovery to differentiate microglial effects on early vs late survival of newborn neurons. We will combine bulk labeling of entire cell cohorts and labeling of single newborn neurons to allow both quantitative as well as functional/qualitative analysis. Behavioral tests will confirm clinical relevance of newborn neurons for functional recovery after CA. Confirmation that microglia are mandatory for successful neurogenesis after CA will introduce another facet of the microglial response to ischemic injury, which is different from the toxic response driving neuronal death. This will create a paradigm shift, as future therapeutic interventions will need to carefully consider these two opposing capacities, and aim to suppress one without interfering with the other. The proposed experiments are highly relevant to the mission of NINDS, as they increase understanding of fundamental processes in the injured brain that will lead to therapeutic advances to improve recovery after CA.